2. Technical Field
This invention relates to a propulsion system for a vehicle or machine such as an aircraft, boat, wind generator, and the like, characterized by two engines that drive two coaxial propellers, props or fans, and particularly to such a system employing a transmission that allows each of the propellers, props, or fans to be exclusively driven by a separate one of the engines.
3. Background Art
Twin engine aircraft provide many advantages over single engine aircraft. First, the use of two engines can increase the overall power of the aircraft, allowing for higher thrust levels and faster air speeds. Next, two engines provide a redundancy that enhances the safety of the aircraft. Should one of the two engines fail, the remaining engine is typically sufficient to fly the plane and land safely. For these reasons, aircraft used for carrying passengers for hire must under certain conditions have at least two engines.
Most current twin engine propeller driven airplanes mount an engine on each wing. However, wing mounted engines reduce aerodynamic efficiency. They also require complex structures and expensive duplicate installations of components and systems. Wing-mounted engines further produce dangerous control problems resulting from sudden asymmetrical thrust should one of the engines fail.
One solution to the problems presented by mounting the engines on the wings of a twin engine aircraft has been to power a single propeller with both engines. This single propeller unit is typically mounted on the nose of the airplane with both engines arranged in a side-by-side configuration just behind the propeller unit. Most of these twin-engine, single propeller propulsion systems were design so that in the event of a failure of one of the engines, the remaining engine could still power the propeller, albeit at reduced power. Thus, the twin-engine, single propeller system retains the aforementioned advantages of a twin engine aircraft, namely increased power and safety, while eliminating the disadvantages associated with the wing-mounted twin engine configuration. However, such systems do not realize the benefits associated with a so-called coaxial, dual-propeller design.
Current coaxial, dual-propeller modules typically employ a single engine to power two coaxial, counter-rotating propellers. Although, at least one system has been proposed where two engines are used to drive a single mechanism that, in turn, drives both propellers. Such systems are advantageous because they are capable of providing an enhanced thrust efficiency. A modern multi-blade propeller creates considerable thrust used to propel the airplane through the air. However, a substantial component of swirl is also imparted to the slipstream generated by the propeller. This swirl component degrades the propulsive efficiency of the propeller. To counteract the swirl component and increase the efficiency of the propulsion system, a second, coaxial propeller is positioned just behind the first. This rear propeller rotates in the opposite direction as the forward propeller, thereby imparting a component of swirl to the slipstream coming from the forward propeller that has the opposite sense from the swirl component created by the forward propeller. The result is a net swirl component that is nil or relatively small. Consequently, the propulsive efficiency is increased. Studies have shown the increased propulsive efficiency to be on the order of nine (9) percent. This represents a significant increase in power and speed capabilities of the aircraft, as well as an opportunity for significant fuel savings.
However, single engine, coaxial, dual-propeller propulsion systems have the inherent disadvantage of all single engine designs. Namely, if the engine fails both propellers are rendered useless. Further, even with the twin engine system mentioned above, the propellers would stop should the single mechanism used to rotate both propellers fail.
Current single-engine, coaxial, dual-propeller propulsion systems are not simple devices, either. Complex gearing arrangements are needed to drive the propellers in opposite directions. In addition, it is desirable that the pitch of the blades on each propeller be independently controllable. Typically, all the blades of each propeller are independently capable of being rotated in unison from a full reverse thrust position which provides maximum thrust, to a so-called feathered position wherein relatively little on no reverse thrust is produced but drag is minimized and turning of the engine (i.e. windmilling") by the propeller during flight is prevented. Pitch positions in-between full reverse thrust and feathered allow the thrust and speed of the aircraft to be regulated while maintaining a constant engine or propeller speed. It is noted that the pitch of the rear propeller is reversed from that of the forward propeller so that it produces a rearward directed thrust like the forward propeller even though it is rotating in the opposite direction. It may also be advantageous to reverse the pitch of the respective propellers so as to produce a forward thrust. This allows the propellers to be used as a brake, both during flight and upon landing, as well as to assist in maneuvering the aircraft while on the ground. Independent control of the blade pitch of each propeller allows the pitch of the rear propeller to be adjusted to compensate for the effect of the slipstream from the forward propeller so that the rear propeller can operate in that slipstream efficiently and contribute to the overall thrust produced by the system. However, independent pitch control for each propeller further complicates the design of current single-engine, coaxial, dual-propeller systems.
Accordingly, there is a need for a propulsion system for a propeller-driven aircraft which enjoys all the benefits of a single-propeller, twin engine design but which also provides the enhance propulsive efficiency available with a coaxial dual-propeller system and which is not susceptible to failure should any single mechanism that rotates the propeller(s) fail. In addition, such a new system would preferably provide independent control of blade pitch of each of the coaxial propellers without the complexity of current systems.
In addition to propeller driven aircraft, the above described systems are also sometimes advantageously employed to drive the props of various watercraft varying from large sea-going vessels and submarines to small recreational power boats. These systems can also be employed to drive the fans of a wind generator such as used in wind tunnels. The same need exists for a twin-engine, coaxial, dual-propeller propulsion design with simplified pitch control and no single-component failure mechanism in these applications as well. Therefore, even though the description of the apparatuses embodying the present invention which follow are directed at a propeller-driven aircraft, it is understood that they are equally applicable to prop-driven watercraft and the fans of wind generating equipment as well.